The invention relates generally to a weighing system and method and in more detail to a multiple load cell scale that is capable of simultaneously sensing multiple loads on the scale.
When two or more decks share load cells, such as in long scales used to measure truck weights, one weight and its associated centroid position may be determined for each deck if the individual load cell force measurements are available. This is typically accomplished by determining each load cell""s sensitivity to weight movement on each deck. This sensitivity is then used in conjunction with the centroid position to determine what load (or portion of the load) is being supported by each deck. The individual deck loads and their centroid positions may also be determined, if the total centroid position is available and there is equal movement of the deck loads so that each load remains on a separate deck during movement. A truck""s individual axle loads may be determined as well as its weight on a normal truck scale by utilizing a special multi-load cell output analysis. More than one load may be determined at the same time on a multi-deck scale if the centroids of the loads are defined. This enables a single scale to accomplish what would have required multiple scales, saving on scale cost and installation.
A truck""s individual axle loads must not exceed road limits and drivers are frequently fined if axle weights are found above these limits. It is therefore desired that axle weights be determined as well as weight on a normal truck scale. Scales with more than one load cell can sense centroid position as well as weight when individual load cell forces are available as disclosed in U.S. Pat. No. 5,750,937 to Johnson. Scales with two load cells can only determine the centroid position in one dimension. Scales with three or more load cells can determine the total load""s centroid position in two dimensions. When more than one simply connected decks support the total weight on more than two load cells, the load cells share a portion of the load on each deck. The individual deck loads are not isolated to a set of load cells that are unaffected by another deck load. Thus, the load on each deck is not directly equal to the sum of the forces on any of the individual load cells supporting the deck. The load cell forces are dependent on more than one deck load and a direct solution using only the force measurements is not possible.
Accurate axle weights of trucks have been measured on multiple scales in the past as a general rule. This method is accurate, but requires a scale for each axle on a truck, increasing the cost of the facility. Weighing the total load as the axles were added to the scale and then using the incremental changes in weight to determine the axle weights also was used. This only works if the truck is moving slowly without acceleration or the truck stops after each axle boards the scale and there are level approaches to the scale without bumps. Axle loads can also by measured by tipping the vehicle according to U.S. Pat. No. 5,753,865 to Lechman. This requires two scales with an unusual arrangement. The prior art also describes movable scales such as that disclosed in U.S. Pat. No. 5,583,777 to Power that allow various configurations of wheel loads to be measured. These scale measure the load centroid but require multiple scales.
U.S. Pat. No. 4,667,757 to Johnson discloses a method of determining the spacing between axles using a scale with only two load cells and requires that time measurements be timed to occur when the second axle boards the scale. Centroid measurements are not available in this scale operation. U.S. Pat. No. 5,004,058 to Langford and U.S. Pat. No. 4,804,052 to Griffen address the problem of load position compensation provided a multiple load cell scale. These scales provide individual load cell force information for scale diagnosis and calibration, but do not provide the means of measuring ether centroid positions or multiple loads. U.S. Pat. No. 5,750,937 to Johnson provides a multi-load cell forcing apparatus for measuring the force and its position, but does not provide the means to measure multiple loads and positions.
Thus, it is desirable to provide a multiple load sensing multi-load cell scale that overcomes the above problems and limitations of the prior art and it is to this end that the present invention is directed.
The multiple load sensing multi-load cell scale in accordance with the invention provides many advantages over conventional systems and provides capabilities not achievable with the typical systems. For example, the invention provides improved utility to scales with multiple deck s that share load cells near supports so that more than one load may be determined at the same time on a single scale. In more detail, the scale in accordance with the invention uses the sectional sensitivities to load movement on the scale deck s, load position and individual normalized load cell outputs to provide separate load magnitude measurements on these individual deck s.
According to the present invention, more than two load cells associated to form a single scale provide outputs that are converted to individual digital representations of the relative force magnitude on each. These magnitudes are related to the effective location of each load cell to provide the load centroid location on the scale when the sum of the products of the effective location and magnitude of each is divided by the sum of the magnitudes. The scale in accordance with the invention provides the sensitivity of each load cell to load movement on each deck. In particular, the portion of the total scale load on each deck is resolved when these deck loads are moved equally. Individual deck loads are determined by taking advantage of the fact that the change in total load magnitude across a load bearing section of the scale is the result of the product of the sum of products of sectional moment sensitivity, the deck load and the distance the load moves on the deck to cause the change. Equations for section forces supporting each deck provide a set of simultaneous interdependent equations that have a linear solution for the set of individual deck loads.
The scale in accordance with the invention also provides, on the above scale, a measure of load position on each deck derived from the magnitude of the deck loads, sectional moment sensitivities, relative load cell locations and intercepts for force versus position dependencies. The intercepts are the effective locations where the force due to a sectional sensitivity is zero. These intercepts are determined by moving a load to a place on one deck at a time and solving for the positions for null force on each load cell section. The positions of the deck loads are then solved for by utilizing the set of equations for the sectional forces with zero intercept compensations equal to the sum of the products of the deck load, deck load position, and moment sensitivity.
The scale in accordance with the invention also provides, on the above scales, a measure of deck load width position on each deck derived from load cell outputs, sectional moment sensitivities, relative load cell locations and zero intercepts for force versus position dependencies. The scale in accordance with the invention also provides another object of the invention, which is to provide the portion of the total scale load on each deck when the position of deck load on each deck between supports is known. Individual deck loads are determined by taking advantage of the fact that the total load magnitude across a load bearing section of the scale is the result of the sum of products of sectional moment sensitivity, the deck load and the distance the load is from the sectional force intercept. Equations for section forces supporting each deck provide a set of simultaneous interdependent equations that have a linear solution for the set of individual deck loads.
Thus, in accordance with the invention, a force sensing apparatus is provided. The force sensing apparatus comprises an operating surface having one or more decks connected together for receiving an applied force from one or more objects and the applied force has a position relative to the operating surface and a magnitude. The force sensing apparatus further comprises a plurality of load cells located adjacent an edge of a deck, each load cell receiving at least a portion of the applied force on the operating surface and providing a force value representative thereof. The force sensing apparatus further comprises a control system that simultaneously determines, based on the force values from the load cells, the location and weight of the one or more objects and outputs one or more signals corresponding to the magnitude of the one or more objects on the force sensing apparatus.
In accordance with another aspect of the invention, a force sensing apparatus is provided comprising a load receiving means having an operating surface with one or more decks connected together that receives an applied force from one or more objects wherein the applied force has a position relative to the load receiving means and a magnitude. The force sensing apparatus further comprises load cell means having a plurality of load cells located adjacent an edge of a deck wherein each load cell receives at least a portion of the applied force on the operating surface and provides a force value representative thereof The apparatus further comprises means for simultaneously determining, based on the force values from the load cells, the location and weight of the one or more objects, and means for outputting one or more signals corresponding to the magnitude of the one or more objects on the force sensing apparatus.
In accordance with another aspect of the invention, a force sensing apparatus for determining the total weight and axle loads of a truck is provided. The apparatus comprises an operating surface having one or more decks connected together for receiving an applied force from the truck wherein the applied force has a position and a magnitude corresponding to each axle of the truck. The apparatus further comprise a plurality of load cells located adjacent an edge of a deck wherein each load cell receives at least a portion of the applied force on the operating surface and providing a force value representative thereof The apparatus further comprises a control system that simultaneously determines, based on the force values from the load cells, the location and load for each truck axle and the total weight of the truck and outputs one or more signals corresponding to the magnitude of the total weight of the truck and the load on each axle.